This invention relates to zirconia ceramics superior in mechanical strength, hardness and toughness and a method for producing same. More particularly, it relates to zirconia ceramics superior in mechanical strength, hardness and toughness which is especially suitable as mechanical structural materials and which contain cerium oxide (sometimes referred to as "CeO.sub.2 " hereinafter) and magnesium oxide (sometimes referred to as "MgO" hereinafter) as stabilizers and a method for producing same.
It has been well known to use CeO.sub.2 or MgO as a stabilizer for zirconium oxide (sometimes referred to as "ZrO.sub.2 " hereinafter).
For example, when CeO.sub.2 is contained in an amount of 20 mol % or more for ZrO.sub.2, a zirconia ceramic comprising tetragonal phase and cubic phase can be obtained, when CeO.sub.2 is contained in an amount of 16-20 mol %, a completely stabilized zirconia ceramic comprising tetragonal phase crystal can be obtained and when CeO.sub.2 is less than 16 mol %, an unstabilized zirconia ceramic comprising only monoclinic phase at room temperature can be obtained. [cf. "Journal of Materials Science", 17 (1982), page 256, FIG. 1]. However, all of these products are not sufficient in mechanical strength and hardness, although high in toughness.
Furthermore, zirconia ceramics comprising ZrO.sub.2 containing MgO are also known, but in this case, tetragonal phase precipitated at 1240.degree.-1400.degree. C. is decomposed into monoclinic phase and MgO while cooling.
Therefore, in the case of magnesia-partially stabilized zirconia ceramics, the mixture is first fired to obtain cubic phase solid solution and rapidly cooled at a suitable cooling rate to precipitate tetragonal phase in the cubic grain matrix. [cf. "Journal of American Ceramic Society", vol. 50, no. 6 (1967), pages 288-290].
However, in this case, grain size of cubic phase of the ceramic obtained is large, namely, about 10-100 .mu.m and zirconia ceramics superior in mechanical strength cannot be obtained.
Furthermore, zirconia ceramics prepared using CeO.sub.2 and MgO in combination as a stabilizer is reported in "AUSTCERAM", 86, Aug. 1986, PROCEEDINGS pages 371-378. That is, this literature discloses spongy sintered products having a strength of 700 Mpa in maximum and a toughness of 8 MNm.sup.-1.5 (MeGa Newton/meter 15 ) and a density of 95% or less with crystal grain size of 1 .mu.m or less.
This method is advantageous in that CeO.sub.2 and MgO which are stabilizers are less expensive than Y.sub.2 O.sub.3, but the ceramics obtained by the method cannot be said to be sintered products having well balanced and excellent characteristics such as strength, toughness and hardness which are required as mechanical structural materials.
Under these circumstances, the inventors have made intensive researches in an attempt to obtain zirconia ceramics superior in mechanical strength, hardness and toughness using CeO.sub.2 and MgO which are less expensive than Y.sub.2 O.sub.3 and found that a sintered product which satisfies the above required properties can be obtained when CeO.sub.2 and MgO are used in combination at a specific ratio and when raw material powders are prepared by a specific method and they are shaped and sintered. This invention is based on this finding.